To prevent unauthorized copying of a document composed of image data, an image of a specific pattern (hereinafter, specific image information) such as a woven pattern is added to or combined with the document. This specific image information is also referred to as copy prohibiting information or copy preventing information. Among the method of adding the specific image information are, for example, a method of sneaking the information into a frequency domain such as digital watermarking, a method of adding a bar code, etc., to a non-image area, a method of adding the information such as a watermark to an image area.
The specific image information described above is, in some case, attached to the document in such a state as to be indiscernible to a user and in other case, attached to the document in such a state as to be sufficiently discernible to the user as the specific image information to restrain copying. However, irrespective of whether the specific image information is discernible to the user or not, when the user attempts to copy the document with the specific image information added thereto using a copying machine, etc., the machine performs an operation such as prohibiting the document from being copied or printed or outputting information of “copy prohibited”.
While various types of specific image information are proposed, for example, Japanese Laid-Open Patent Publication No. 2000-307857 discloses printing of a candidate image of a specific pattern in a checkered manner on the back side of a document without impairing readability of the document. It also discloses judging of whether the detected candidate image of the specific pattern is the specific image information, by examining positional arrangement of the candidate image of the specific pattern and prohibiting an operation of copying, etc., in the case of the specific image information. Japanese Laid-Open Patent Publication No. H09-16043 discloses the technology of providing the specific image information divided into a plurality of colors and, at the time of detection, combining extracted results of the plurality of colors into one and accurately detecting the specific image information based on such results.
However, if the specific image information is provided on the back side of an original document, to avoid interference of the specific image information with the image data of the document, as in the Japanese Laid-Open Patent Publication No. 2000-307857, then there is a problem that the document is limited to one of thin paper quality for detecting the specific image information only from one side of the document, that it takes time to detect from both sides of the document, and that the technology is not applicable to the document having the image data on both sides thereof.
If the specific image information is permitted to be present in the document as in the Japanese Laid-Open Patent Publication No. H09-16043, then there will be many cases of erroneously recognizing the image data as the specific image information depending on the image data of the document. For example, when the image data of the original document has the image data similar to that of the specific image information or interferes with the specific image information, the discernibleness could decrease. Furthermore, even if the specific image information is divided into a plurality of colors, interference is caused depending on the original document and detecting accuracy does not necessarily improve depending on the angle at which the document is placed at the time of detection.